The present invention is directed to small-scale Mach-Zehnder interferometer (xe2x80x9cMZIxe2x80x9d) devices and structures having different length arms. The present invention is also directed to both analog and digital optical switches using such asymmetric MZI devices.
An optical network, in its simplest representation, consists of an optical source, a destination, and a matrix of devices (e.g., fiber-optical cables, waveguides, cross-connects, amplifiers, etc.) for causing an optical signal generated by the source to reach a desired destination. Physical and geographic boundaries present no impediment to telecommunication, data communication and computing, all of which may utilize all or part of an optical network. Consequently, the number or sources and destinations, and the combinations of sources and destinations and the communication paths therebetween, may be nearly infinite. Optical switches are used in the optical network for facilitating the routing of an optical signal to its desired destination.
By way of example, FIG. 1 depicts a block diagram of a part of an optical component 1 comprising a plurality of optically interconnected optical devices 3 (e.g., switches, filters, etc.), shown in FIG. 1 as switches. As used herein, the terms xe2x80x9coptical componentxe2x80x9d and xe2x80x9ccomponentxe2x80x9d refer to any and all of a plurality of interconnected devices which may operate using any combination of optical, opto-electrical, and/or electrical technologies and which may be constructed as an integrated circuit. Devices 3 can be optically interconnected by waveguides 5. Various other optical, opto-electrical, and/or electrical devices may also be included in the optical component, as a matter of design choice. As used herein, the terms xe2x80x9copticalxe2x80x9d, xe2x80x9copto-electricalxe2x80x9d, and xe2x80x9celectricalxe2x80x9d devices may include, by way of non-limiting example, lasers, waveguides, couplers, switches, filters, resonators, interferometers, amplifiers, modulators, multiplexers, cross-connects, routers, phase shifters, splitters, fiber-optic cables, and various other optical, opto-electrical, and electrical devices. The optical component 1 and devices 3 depicted in FIG. 1 are merely illustrative.
Although a single wavelength of light can be transmitted through the network, in order to increase the network""s data-carrying capacity it is preferable to transmit multiple wavelengths of light at the same time. This is currently accomplished using techniques known as wave-division-multiplexing (xe2x80x9cWDMxe2x80x9d), dense WDM (xe2x80x9cDWDMxe2x80x9d), and ultra-dense wave-division-multiplexing (xe2x80x9cUDWDMxe2x80x9d).
The ability to separate one optical signal from a plurality of optical signals (or one wavelength from a plurality of wavelengths in an optical signal) propagating within an optical network becomes more important as the number of signals transmitted through a single optical fiber (or waveguide) increases. As optical transmission evolves from WDM to DWDM to UDWDM, and beyond, more and more data contained in a multi-wavelength optical signal is transmitted over the optical network. Optical filters are one component that may be used to extract a desired signal (i.e., a desired wavelength) at a particular point or location in the network and route that signal to its desired destination, while also permitting other signals to continue along the network.
Optical networks transmit data as pulses of light through waveguides in a manner similar to electrical networks, which send pulses of electricity through wiring. Transmitting an optical signal between waveguides, which may occur in various devices employed in an optical network, may require the optical signal to leave one waveguide and propagate through one or more materials (mediums) before entering another waveguide. It is likely that at least one of the devices will have an index of refraction different than the index of refraction of the waveguides (which typically have approximately the same refractive index). It is known that the transmission characteristics of an optical signal may change if that signal passes through materials (mediums) having different indices of refraction. For example, a phase shift may be introduced into an optical signal passing from a material having a first index of refraction to a material having a second index of refraction due to the difference in velocity of the signal as it propagates through the respective materials and due, at least in part, to the materials"" respective refractive indices. As used herein, the term xe2x80x9cmediumxe2x80x9d is intended to be broadly construed and to include a vacuum.
If two materials (or mediums) have approximately the same index of refraction, there is no significant change in the transmission characteristics of an optical signal as it passes from one material to the other. Accordingly, one solution to the mismatch of refractive indices in an optical switch involves providing an index matching or collimation fluid to offset any difference in refractive indices. Consequently, the optical signal does not experience any significant change in the index of refraction as it passes from one waveguide to another.
An example of this approach may be found in international patent application number WO 00/25160. That application describes a switch that uses a collimation matching fluid in the chamber between the light paths (i.e., between waveguides) to maintain the switch""s optical performance. The use of an index matching fluid introduces a new set of design considerations, including the possibility of leakage and a possible decrease in switch response time due to the slower movement of the switching element in a fluid.
In addition, the optical signal will experience insertion loss as it passes between waveguides. A still further concern is optical return loss caused by the discontinuity at the waveguide input/output facets and the trench. In general, as an optical signal passes through the trench, propagating along a propagation direction, it will encounter an input facet of a waveguide which, due to physical characteristics of that facet (e.g., reflectivity, verticality, waveguide material, etc.) may cause a reflection of part (in terms of optical power) of the optical signal to be directed back across the trench (i.e., in a direction opposite of the propagation direction). This is clearly undesirable because the reflected signal will interfere with the optical signal propagating along the propagation direction.
Reflection of the optical signal back across the trench also can create problems if the facets not only are coaxial, but also are parallel to one another. That arrangement forms a Fabry-Perot resonator cavity, which, under the appropriate circumstances, allows for resonance of the reflected signal, in known fashion.
Size is also an ever-present concern in the design, fabrication, and construction of optical components (i.e., devices, circuits, and systems) for use in optical networks. It is strongly desirable to provide smaller optical components so that optical devices, circuits, and systems may be fabricated more densely, consume less power, and operate more efficiently.
Currently, optical switches can be constructed using a directional coupler or a Mach-Zehnder interferometer (xe2x80x9cMZIxe2x80x9d), as is generally known in the art. Mach-Zehnder interferometers are known devices which take an input optical signal, split the signal in half (generally, in terms of optical power), direct the split signals along different optical paths, apply a phase shift to one of those split signals, recombine the signals and then feed those combined signals as a single signal to an output. The amount by which the phase of one of the signals is changed will, in known fashion, affect the nature of the output signal.
Conventional Mach-Zehnder interferometers shift the phase of light traveling along one of the interferometers in one of several ways. In one such way, one of the two interferometer arms is longer than the other, and this results in an inherent relative phase shift between the two signals propagating through the interferometer arms; the light traveling through the longer arm has its phase shifted relative to the light passing through the other arm. This can be considered a biasing phase shift.
The inherent relative phase shift may be xe2x80x9ctunedxe2x80x9d using either the electro-optic or electro-thermal technique to apply a further phase shift to the optical signal traveling through the associated arm. In this case, the overall phase shift applied to the optical signal traveling through the associated arm is the sum of the biasing phase shift and the applied phase shift. If the electro-optic effect alone is used, one of the interferometer arms is made from a medium having an index of refraction which changes in the presence of an applied electrical field. Similarly, if the electro-thermal effect is used, the interferometer has an arm made from a medium having an index of refraction that changes as the temperature of the material changes. In each of these devices, changing the index of refraction of one of the interferometer arms is comparable to changing that arm""s optical length, and results in a relative phase shift between the two split signals.
In the electro-optic and electro-thermal type devices, the conditions for effecting optical switching in a device using a MZI, which operates by introducing a phase shift of up to xcfx80 (i.e., 180xc2x0) into at least a part of the optical signal, are defined by the equation:                     Δφ        =                  π          =                                                    2                ⁢                π                            λ                        ⁢            Δ            ⁢                          xe2x80x83                        ⁢            nL                                              (        1        )            
where xcex94xcfx86 is the maximum possible phase shift of xcfx80xcex is the wavelength of the optical signal propagating in the device, L is the actual length of the device, and An is the change in refractive index effected by the application of a carrier signal, electrical field, or change in temperature to the device. Since the change in refractive index typically achievable for current optical devices is on the order of approximately 10xe2x88x923, the actual length of the device needed to introduce a maximum phase shift of xcfx80 must be at least 1 mm, and preferably longer. However, to achieve large-scale density integration, the actual length L must be reduced without sacrificing the ability to effect a xcfx80 phase shift in an optical signal. Those two requirements are mutually exclusive.
If the phase is to be applied using a MZI device having different length arms, the light traveling through the longer arm has its phase shifted relative to the light passing through the other arm. This technique cannot, however, be used to make compact optical switches.
There exists a genuine need in the art for compact optical switches that can effect a 0-xcfx80 phase shift and which overcome the above-described shortcomings of the prior art. Preferably, such switches would combine small size and high actuation speed with low power consumption.
The present invention is directed to an analog optical switch having a MZI with a moveable phase shifter in one interferometer arm suitable for use in an optical network.
More particularly, this invention is directed to improved analog Mxc3x97M switches which employ asymmetric Mach-Zehnder interferometers to control optical signals. As already explained, asymmetric MZI devices operate by dividing an input optical signal into two signals, applying a biasing phase shift and possibly a variable phase shift to just one of those signals, and then recombining the two signals. The output will depend upon the magnitude of the total phase shift applied, which will in part be a function of the MZI arm asymmetry. As noted previously, known switches of this type are larger than desired because the asymmetric MZI devices used therein operate using techniques which thwart miniaturization.
Switches according to the present invention differ from known 1xc3x971 and 2xc3x972 optical switches because of the unique asymmetric MZI provided in accordance with the present invention. An asymmetric MZI constructed in accordance with embodiments of the present invention includes a phase shifter in one of the two asymmetric arms of the interferometer. The phase shifter is selectively moveable into and out of an optical path defined by and through the interferometer arm so as to introduce a phase shift into an optical signal propagating in and through that interferometer arm. The phase shift applied by the phase shifter can be either a variable phase shift of 0-180xc2x0 (xcfx80) or a fixed phase shift of between 0-180xc2x0 (xcfx80). This arrangement dramatically reduces the size of the MZI as compared with conventional optical switches, which may employ the electro-optic, electro-thermal or asymmetric arms to introduce a phase change in an optical signal. An asymmetric MZI using a phase shifter in accordance with this invention is more compact than an asymmetric MZI which uses known techniques.
The present invention is particularly applicable to optical switches that are formed on integral planar optical substrates. Generally speaking, an integrated planar optical substrate refers to a relatively flat member having a supporting substrate and a number of layers of different materials formed thereon. The substrate and the different materials have particular optical qualities so that optically useful structures such as waveguides can be formed on the supporting substrate by suitable shaping or other processing. Such optical switches may be more compact and more rapidly actuated than comparable known devices.
As explained in greater detail below, this invention involves phase shifters constructed using small-scale fabrication techniques. This invention also encompasses phase shifters made using other fabrications techniques which result in comparable devices.
The present invention takes advantage of the extremely small mechanical actuators which can be assembled using small-scale fabrication techniques, and so significantly reduces the room needed on a chip for optical switches. These more compact switches require less chip space and so provide for denser integration of a plurality of optical devices in an optical component. This invention also takes advantage of the strong photon confinement properties of small-scale waveguides, such as are disclosed in U.S. Pat. Nos. 5,878,070 and 5,790,583. Together these developments facilitate construction of optical devices that provide the benefits and advantages of the present invention.
One embodiment of the present invention involves a Mach-Zehnder interferometer having a single input, a single output, and asymmetric first and second arms extending along an optical path direction of the interferometer. One arm, preferably the longer arm, has a phase shifter disposed therein. Alternatively, the phase shifter could be located in the shorter arm. When the phase shifter is actuated an optical signal propagating through the arm having the phase shifter will experience a phase shift relative to an optical signal propagating through the other arm. This phase shift is in addition to the inherent biasing phase shift which results from the different lengths of the two interferometer arms. The phase shifter can apply either a variable or a fixed phase shift.
In accordance with the present invention, an asymmetric MZI may be constructed with a selectively moveable phase shifter in one of the two different-sized interferometer arms. hat phase shifter may be moved into and out of an optical path defined by and through that interferometer arm so as to introduce a phase shift into an optical signal propagating in and through that arm. In so doing, the phase shifter changes the optical length of that arm, when compared with the optical length of the other interferometer arm. The total relative phase shift between the optical signals traveling in the two interferometer arms is the sum of the biasing phase shift and the applied phase shift. The phase shifter may be generally wedge-shaped, rectangular, square, stepped (on one or both sides), or other shapes, provided that such shapes may be utilized to introduce a phase shift into the optical signal.
While it is generally known to provide an asymmetric MZI as an element of an optical switch, an asymmetric MZI constructed in accordance with the embodiments of the present invention provides significant advantages over prior art asymmetric MZI devices and optical switches. For example, the minute size of the MZI enables construction of smaller optical switches that consume less on-chip real estate. The power requirements of the MZI to effect a desired phase change in an optical signal are also significantly reduced when compared with prior art MZI devices.
The invention accordingly comprises the features of construction, combination of elements, and arrangement of parts which will be exemplified in the disclosure herein, and the scope of the invention will be indicated in the claims.